In the manufacture of paper, an aqueous slurry of about 99% by weight of water and 1% by weight of cellulosic fibers and other papermaking constituents is deposited from a headbox onto a moving forming fabric, or in between two moving forming fabrics on a two-fabric papermaking machine. The web is initially formed and partially drained in the forming section, and is transported downstream where it is consolidated and dried by known means, such as conventional press dewatering in the press section, and evaporative drying in the dryer section. However, if the finished sheet is intended to have liquid absorbency properties, for end uses such as for tissue or towel, improved results can be obtained through the use of a through-air drying (TAD) instead of the conventional press and drying methods.
Water removal in a TAD process occurs as air is passed through the web and through the TAD fabric being used to support and convey the web through the TAD dryer section. This air movement molds the web to the surface topography of the TAD fabric, while removing most of the remaining moisture. The molding creates a more three dimensional web, thus increasing the thickness (known as bulk) of the finished web, which improves the efficacy of the finished product for applications such as tissue or towel. One means of imparting a desired topography to a TAD fabric is to apply a polymeric resin with precision in a desired pattern to the paper contacting, or paper side (PS), surface of the fabric.
Polymeric resin coated fabrics are well known, and have been described for example in U.S. Pat. No. 4,514,345 to Johnson et al., and U.S. Pat. Nos. 4,528,239, and 4,529,480 to Trokhan. Such resin coated structures generally comprise a reinforcing structure, referred to herein as a Acarrier fabric@, onto which a functional polymeric resin is deposited and subsequently pattern cured, for example by using a light source of activating wavelength through a mask. The resulting TAD fabric will generally have a macroscopically monoplanar patterned resinous network, either semicontinuous or discontinuous, on one surface.
The physical properties of the carrier fabric onto which the polymeric resin is to be deposited, and the balancing interaction between these properties, are critical to the effectiveness of the resultant TAD fabric. Some of the factors which affect the selection of these physical properties include the following:
Firstly, a high amount of projected open area, being the amount of open space per unit area projected through a fabric when viewed perpendicularly to the plane of the fabric, is required. Thus a woven carrier fabric must have a relatively open structure, in order to provide sufficient void volume for the polymeric resin in the finished TAD fabric, and to allow for the passage of sufficient air from the TAD dryer drum through the fabric and the web. If the carrier fabric is a closely woven structure, it will tend to become filled when the polymeric resin is applied, thus closing or unduly restricting the air passages.
Secondly, the carrier fabric must be dimensionally stable, and capable of resisting in-plane distortion such as is encountered when the fabric passes over bowed or spreader rolls in the papermaking machine. If the fabric does not have this stability, it may become narrowed or lengthened along its centre line, or suffer from creasing, or undulations across its width, any of which may impair its runnability and effectiveness. Such variations in the otherwise smooth planar nature of the fabric may cause localized variations in the paper product being conveyed by the fabric, which can lead to sheet breaks and a disruption in the operation of the papermaking machine.
Thirdly, the carrier fabric must be capable of being seamed effectively, preferably by a relatively narrow woven seam, which must be of sufficient strength to resist the longitudinal i.e. machine direction (MD) tensile forces to which the fabric is exposed. Typically, when a fabric such as this is prepared for a woven seam, the warp and weft yarns at the opposing fabric ends are unravelled and then rewoven into each other to form a seam region, usually having a width of between 5 and 12 inches. This woven seam must possess sufficient tensile strength so that the warp yarns resist sliding apart when the fabric and the seam are exposed to the expected MD tensile forces during use, which are typically up to 50 or 60 pounds per linear inch. One means of ensuring sufficient tensile strength at the woven seam is to impart sufficient crimp to the warp yarns during the fabric weaving, so that the yarns will have a greater resistance to sliding apart when the fabric is in use, and the seam will tend to have greater resistance to opening under longitudinal stress. If the crimp is insufficient for a given seam width, the warp yarns will tend to slide apart from the weft yarns, and the seam is more likely to fail. One means of ensuring that the warp yarns are crimped sufficiently to resist seam failure is to weave the fabric according to a plain weave pattern, which maximizes the number of crimps per unit length of the warp yarn.
Designers of carrier fabrics such as those of the prior art have been faced with the difficulty of meeting and reconciling these and other criteria. In particular, for an effective TAD fabric, it is necessary to provide a weave structure which has a high open area, while at the same time is woven to a pattern which provides sufficient yarn crimp to provide stability in each dimension, and to provide a durable seam.
Single layer TAD fabrics are well known and have been described in U.S. Pat. Nos. 5,839,479 and 5,853,547 to Wright et al. These patents teach that sheet bulk is enhanced by the use of cross direction (CD) yarns of alternating large and small diameters, the weave pattern in each case resulting in paper bulking depressions on the PS surface.
Double or multilayer fabrics have also been developed for use as TAD fabrics. For example, U.S. Pat. Nos. 5,496,624 and 5,840,411 to Stelljes each describe a double layer fabric which can be subjected to resin coating by means of a resinous pattern layer cast over the PS surface.
It has further been found that sheet bulk can be enhanced by the use of multilayer fabrics including vertically stacked yarns. For example in PCT Publication No. WO 03/006732 to Johnson et al., two sets of weft yarns are substantially vertically aligned, to urge the warp yarns into greater prominence on the PS surface; and alternatively, two sets of warp yarns can also be vertically stacked.
It is known to use pairs of either or both warp and weft yarns to bind together the layers of double or multilayer forming fabrics. For example, U.S. Pat. No. 5,826,627 to Seabrook et al. discloses a forming fabric including pairs of intrinsic weft binder yarns, which are weft yarns that contribute to the structure of both the PS and MS fabric surfaces, and also serve to bind these fabric layers together. However, other “regular” weft yarns are interspaced with the intrinsic weft binder yarns of this fabric.
Further, Application No. PCT/EP01/09398 to Odenthal shows a composite forming fabric comprising a PS layer having a plain weave pattern, formed of intrinsic weft pairs, one member of each pair also serving to bind together the PS and MS layers. However, in each pair the other member does not serve to bind the two layers together, and the long MS warp floats would contra-indicate use of this fabric as a TAD carrier fabric.
It has been found that an effective TAD carrier fabric can be successfully manufactured using a weave pattern in which all the weft yarns are arranged as pairs of intrinsic binder yarns, and are woven so as to bind together the warp yarns of each of the PS and MS layer, which are arranged in vertically stacked pairs. By the selection of an appropriate weave pattern, a high open area can be provided, enabling effective resin coating, and at the same time providing a dimensionally stable fabric having sufficient crimp in the warp yarns to allow for durable seaming.